Method and Apparatus for Maintaining Memory Data Integrity in an Information Handling System Using Cache Coherency Protocols

ABSTRACT

An information handling system includes a processor integrated circuit including multiple processors with respective processor cache memories. Enhanced cache coherency protocols achieve cache memory integrity in a multi-processor environment. A processor bus controller manages cache coherency bus interfaces to master devices and slave devices. In one embodiment, a master I/O device controller and a slave I/O device controller couple directly to the processor bus controller while system memory couples to the processor bus controller via a memory controller. In one embodiment, the processor bus controller blocks partial responses that it receives from all devices except the slave I/O device from being included in a combined response that the processor bus controller sends over the cache coherency buses.

TECHNICAL FIELD OF THE INVENTION

The disclosures herein relate generally to information handling systems,and more particularly, to cache coherency protocols in multi-tasking andmulti-processor systems.

BACKGROUND

An information handling system (IHS) may include multiple processors forprocessing, handling, communicating or otherwise manipulatinginformation. Each processor may itself include multiple processor coresthat work together to process information. A processor or processor coremay include processor functional units such as a stack pointer, aprogram counter, a fetch and decode unit, an issue and execute unit,register files, and other processor units. The processor may furtherinclude one or more caches or cache memories for storing information foraccess by the processor or processor core during normal memory load andstore operations. A system memory may be accessible by multipleprocessors within the IHS. A processor or other cache may store datainformation local to each processor to provide faster access to copiesof memory data such as system memory data.

A cache is a storage mechanism that provides local duplication of memorydata values that an IHS stores in other locations such as system memory,register files, or other storage locations. For example, a processor orprocessor core may employ a local or nearby cache memory for fast accessto memory data values. More simply put, the cache is a temporary storagearea where data resides that processors or other devices may frequentlyaccess.

Caches increase the performance of software applications that frequentlyaccess the same data locations in system memory. System memory typicallyemploys dynamic random access memory (DRAM). Cache memory typicallyemploys static random access memory (SRAM) that is generally much fasterthan DRAM. Thus, memory accesses to cache memory are usually much fasterthan memory accesses to system memory.

When a device such as a processor desires access to a particular memorydata value, it first checks the cache memories within the IHS for thesame data value. If the processor finds a cache entry with a tag, oraddress identifier, that matches the particular desired memory datavalue, then the processor accesses that particular memory data value inthe fast cache memory instead of the slower system memory. A cache datavalue found condition represents a “cache hit”. For example, a webbrowser program may execute in a particular processor of an IHS. Theparticular processor may check local cache memory in an attempt to finda copy of the contents of a web page of a particular universal resourcelocator (URL) that the web browser program requests. In this example,the URL is the tag, and the contents of the web page are the memorydata. A cache hit occurs when the processor finds the requested web pagedata in cache memory.

Alternatively, if the particular processor does not find the requestedweb page data in a local cache memory, the result is a “cache miss”.Often the data that the particular processor requests will be part ofthe cache the next time the particular processor requests that samedata. One type of cache is a “write-back” cache. A write-back cache mayhold the most recent value of a memory location without immediatelysending the same data to system memory. A processor may write data to awrite-back cache before the processor initiates a write of that samedata to the system memory or other backup memory location. In awrite-back cache, the processor may perform multiple writes withdifferent data each time. The processor may also read from thewrite-back cache multiple times before the write-back cache initiates awrite to system or backup memory.

Caches achieve a reduction in overall memory processing time by allowingpreviously read data from system memory, or other data that processorswrite, to be readily available to processors during memory read andwrite operations. If data is available in a cache within the IHS,processors can access this cache rather than accessing slower systemmemory. As multiple caches become available within an IHS, multiplecaches may store multiple copies of the same system memory data. As thesize, count, and complexity of cache memories increase, the complexityof managing conflicts among duplicate copies of memory data alsoincreases.

What is needed is a method and apparatus that addresses the problemsassociated with managing multiple cache memories in a multi-tasking andmulti-processor IHS environment as described above.

SUMMARY

Accordingly, in one embodiment a method of maintaining cache coherencyin a processor integrated circuit (IC) is disclosed. The method includesproviding the processor IC with a plurality of master processor coresthat each communicate with a respective cache memory. The method alsoincludes providing a processor bus controller (PBC) that couples to themaster processor cores, the PBC being configured to couple to a systemmemory indirectly via a memory controller, the PBC being furtherconfigured to couple directly to a master I/O device controller and aslave I/O device controller. The method further includes sending, by amaster processor core or a master I/O device, an initial command datarequest that references an address range in the system memory, thusdetermining a referenced address range. The method further includesreceiving, by the PBC, the initial command data request and in responsesending a reflected command to any master processor core, system memoryand I/O slave device that the PBC determines to be within the referencedaddress range. The method also includes sending in response to thereflected command, by the master processor cores, system memory, andslave I/O device in the referenced address range, respective partialresponses to the PBC. The method further includes blocking, by the PBC,the partial response of all devices except the slave I/O device frominclusion in a combined response that the PBC sends to master processorcores, master I/O devices and system memory. In one embodiment, theblock step includes blocking, by the PBC, the partial responses of thememory controller, the master processor cores, the master I/O device butnot the slave I/O device from inclusion in a combined response that thePBC sends to master processor cores, the master I/O devices and thesystem memory.

In another embodiment, a processor integrated circuit (IC) is disclosedthat includes a plurality of master processor cores. The processor ICalso includes a plurality of cache memories, each cache memory beingcoupled to a respective master processor core. The processor IC furtherincludes a processor bus controller (PBC) coupled to the plurality ofmaster processor cores. The PBC is configured to couple to a systemmemory indirectly via a memory controller. The PBC is further configuredto couple directly to a master I/O device controller and a slave I/Odevice controller. The PBC is configured to receive an initial commanddata request from a master processor core or a master I/O device,wherein the initial command data request includes a referenced addressrange in the system memory. In response to the initial command datarequest, the PBC sends a reflected command to any master processor core,system memory and I/O slave device that the PBC determines to be withinthe referenced address range. In response to the reflected command, themaster processor cores, system memory and slave I/O device in thereferenced address range, send respective partial responses to the PBC.The PBC blocks the partial response of all devices except the slave I/Odevice from inclusion in a combined response that the PBC sends tomaster processor cores, master I/O devices and system memory. In oneembodiment, the PBC blocks the partial responses of the memorycontroller, the master processor cores, the master I/O device but notthe slave I/O device from inclusion in a combined response that the PBCsends to master processor cores, the master I/O devices and the systemmemory.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only exemplary embodiments of theinvention and therefore do not limit its scope because the inventiveconcepts lend themselves to other equally effective embodiments.

FIG. 1 is a block diagram of a conventional information handling systemwith multiple processors, caches, and cache coherency protocolcapability.

FIG. 2 is a flow chart that depicts an example of a cache coherencyprotocol method of the system of FIG. 1.

FIG. 3 is a block diagram of the disclosed information handling systemwith multiple processors, caches, and enhanced cache coherency protocolcapability.

FIG. 4 is a flow chart that depicts an example of an enhanced cachecoherency protocol method that the system of FIG. 3 employs.

DETAILED DESCRIPTION

Cache coherency protocols refer to methodologies that manage theintegrity of data in caches such as those that reside in a processor orprocessor core. A particular processor core may include multiple cachesthat support the memory read and write requirements of that particularprocessor core. Cache coherency protocols, such as the modified, shared,invalid (MSI) protocol, the modified, exclusive, shared, invalid (MESI)protocol, and the modified, owned, shared, invalid, (MOSI) protocol, andother protocols are crucial to the accuracy, integrity, and efficientmanagement of multiple cache, multiple processor, and multipleprocessing task systems.

IHSs may include master devices such as master processors, master I/Odevices, as well as other master devices. A particular master device mayrequest data from a memory location external to itself but within theIHS. Master devices often employ local or on-board caches for storingmemory data for efficient and fast access. IHSs also include slavedevices such as read only memories, peripherals, slave I/O devices, andother slaves. Slave devices do not typically have the ability to accessmemory data external to themselves. A master device or other controllingdevice within an IHS typically governs the actions of a slave device.

IHS may include a snoop device or snooper that may include a cachememory. The snoop device maintains the integrity of the data in itscache memory. Snoop devices may be master devices, slave devices, orother devices that monitor cache memory data integrity of other deviceswithin the IHS. An IHS may include a processor bus controller (PBC) thatprovides snoop devices with information to allow snoop devices to trackthe integrity of their local caches.

Unfortunately, cache date integrity problems may arise when multiplemaster devices such as processors and processor cores maintain cacheswith a common memory resource or backup such as a system memory, in asystem without a cache coherency mechanism. One common problem occurswhen one cache of multiple caches contains data from a particular IHSmemory location and another cache of multiple caches contains an oldercopy of that same IHS memory location. The older copy of that same IHSmemory location may be an invalid data copy. The cache with the invaliddata copy may be unaware that the data is not valid. When another masterdevice requests a copy of that memory data, the cache with invalid datacould offer the information as valid. Cache coherency protocols managesuch conflicts and maintain consistency among multiple caches and systemmemory.

Cache “snooping” is one methodology that snoop devices with caches useto support cache coherency protocols. Snooping involves a processwherein individual caches monitor address lines or address referencedata for memory accesses matching their respective cache stores. When asnoop device observes a memory data write operation consistent with itslocal caches, that snoop device invalidates its own copy of the memorydata. An arbitrator, memory data bus controller, network switch, orother device such as a PBC, selects commands to be broadcast by thearbitrator to all snoop devices. The arbitrator or other device selectsthe commands to be broadcast as reflected commands. Snoop devicesmonitor the resultant broadcasts to determine what action, if any,should be taken by the snoop device within the snoop device localcaches.

FIG. 1 shows a conventional information handling system IHS 100 withcache coherency protocol capability that includes many structuresintegrated on a common semiconductor chip 105. IHS 100 includes multipleprocessors and multiple caches or cache memories. IHS 100 includes aprocessor A 110 with a processor core 112. Processor core 112 is a“master processor core” since processor A 110 is a master device of IHS100. Processor core 112 couples to an L1 cache 115 that couples to an L2cache 117 within processor A 110. In conventional processor systems, L1cache 115 is typically smaller than L2 cache 117 and provides processorcore 112 with the closest and fastest memory data in comparison withother data stores available to processor core 112.

If processor core 112 requests particular memory data from L1 cache 115,and L1 cache 115 returns a “cache hit”, the particular memory data isavailable from L1 cache 115. However, if processor core 112 requestsparticular memory data from L1 cache 115, and L1 cache 115 returns a“cache miss”, the particular memory data is not available from L1 cache115. Processor core 112 continues searching by passing the data requestthrough L1 cache 115 into L2 cache 117 to attempt to find the particularmemory data. If L2 cache 117 returns a “cache miss”, the particular datais not available from L2 cache 117. In the case wherein the particulardata is not available from any internal cache of processor A 110,processor core 112 must initiate a data request external to processor A110 for the particular memory data. That particular memory data mayreside in another processor, system memory, an I/O device, or any othermemory location inside or outside of chip 105. Processor A 110 is amaster device of IHS 100 because it has the capability of initiatingmemory data requests.

IHS 100 includes a processor B 120 with a processor core 122. Processorcore 122 couples to an L1 cache 125 that couples to an L2 cache 127within processor B 120. L1 cache 125 is typically smaller than L2 cache127 and provides processor core 122 with the closest and fastest memorydata in comparison with other data stores available to processor core122. If processor core 122 requests particular memory data from L1 cache125, and L1 cache 125 returns a “cache miss”, the particular memory datais not available from L1 cache 125. Processor core 122 continuessearching by passing the data request through L1 cache 125 into L2 cache127 in an attempt to find the particular memory data. If L2 cache 127returns a “cache miss”, the particular data is not available from L2cache 127. In the case where the particular data is not available fromany internal cache of processor B 120, processor core 122 must initiatea data request external to processor B 120 for the particular memorydata. That particular memory data may reside in another processor,system memory, an I/O device, or any other memory location inside oroutside of chip 105. Processor B 120 is a master device of IHS 100because it has the capability of initiating memory data requests.

IHS 100 includes a processor C 130 with a processor core 132. Processorcore 132 couples to an L1 cache 135 that couples to an L2 cache 137within processor C 130. In conventional processor systems, L1 cache 135is typically smaller than L2 cache 137 and provides processor core 132with the closest and fastest memory data in comparison with other datastores available to processor core 132. If processor core 132 requests aparticular memory data from L1 cache 135, and L1 cache 135 returns a“cache miss”, the particular memory data is not available from L1 cache135. Processor core 132 continues searching by passing the data requestthrough L1 cache 135 into L2 cache 137 to attempt to find the particularmemory data. If L2 cache 137 returns a “cache miss”, the particular datais not available from L2 cache 137. In the case where the particulardata is not available from any internal cache of processor C 130,processor core 132 must initiate a data request external to processor C130 for the particular memory data. That particular memory data mayreside in another processor, system memory, an I/O device, or any othermemory location inside or outside of chip 105. Processor C 130 is amaster device of IHS 100 because it has the capability of initiatingmemory data requests.

A processor bus controller (PBC) 140 couples to processor A 110 via acommunications interface 145A that includes four cache coherencyprotocol busses, namely an INIT_CMD bus 142A, a REF_CMD bus 144A, aPART_RESP bus 146A and a COMB_RESP bus 148A. Cache coherency protocolINIT_CMD bus 142A is an “initial command” communications bus that amaster device such as processor A 110 uses to communicate with PBC 140.In particular, processor A 110 uses the INIT_CMD bus 142A to communicatea memory data request.

Cache coherency protocol REF_CMD bus 144A is one of multiple REF_CMDbusses that a bus controller such as PBC 140 utilizes to communicatewith all snoop devices such as processor A 110. Snoop devices includeany devices that communicate with PBC 140 and also contain copies of anyparticular data that a master device may require. More specifically, theREF_CMD bus 144A communicates a reflection or copy of data requestcommunications from other master devices within IHS 100. In other words,PBC 140 receives data request commands from one or multiple masterdevices and reflects those commands to one or multiple snoop deviceswithin IHS 100.

In response to the reflected command on the REF_CMD bus 144A, processorA 110 returns a “partial response” on the PART_RESP bus 146A. PBC 140interprets each partial response from snoop devices, such as processor A110, as one partial communication of the total communication orresponses from all snoop devices in IHS 100. The partial responsecommunication includes information pertaining to a memory data requestfrom a particular master device, such as master device processor A 110.PBC 140 may combine the results of partial responses from all snoopdevices within IHS 100 and generate a “combined response” communication.PBC 140 sends the combined response communication on the COMB_RESP bus148A to processor A 110. The particular sequence of events of cachecoherency protocol communications will be described in more detailbelow.

PBC 140 couples to processor B 120 via a communications interface 145Bthat includes four cache coherency protocol busses, namely an INIT_CMDbus 142B, a REF_CMD bus 144B, a PART_RESP bus 146B and a COMB_RESP bus148B. Cache coherency protocol INIT_CMD bus 142B is an initial commandcommunications bus that a master device such as processor B 120 uses tocommunicate with PBC 140. In particular, processor B 120 utilizes theINIT_CMD bus 142B to communicate a memory data request external toprocessor B 120.

Cache coherency protocol REF_CMD bus 144B is one of multiple REF_CMDbusses that PBC 140 employs to communicate with all snoop devices suchas processor B 120. More specifically, the REF_CMD bus 144B communicatesa reflection or copy of communication data requests from other masterdevices within IHS 100. PBC 140 receives data request commands from oneor multiple master devices and reflects those commands to one ormultiple snoop devices within IHS 100. In response to the reflectedcommand on the REF_CMD bus 144B, processor B 120 returns a “partialresponse” on the PART_RESP bus 146B. The partial response communicationincludes information pertaining to a memory data request from aparticular master device. PBC 140 may combine the results of partialresponses from all snoop devices within IHS 100 and generate a combinedresponse communication. Processor bus controller sends the combinedresponse communication on the COMB_RESP bus 148B to processor B 120.

PBC 140 couples to processor C 130 via a communications interface 145Cthat includes four cache coherency protocol busses, namely an INIT_CMDbus 142C, a REF_CMD bus 144C, a PART_RESP bus 146C and a COMB_RESP bus148C. Cache coherency protocol INIT_CMD bus 142C is an initial commandcommunications bus that a master device such as processor C 130 uses tocommunicate with PBC 140. In particular, processor C 130 uses theINIT_CMD bus 142C to communicate a memory data request external toprocessor C 130.

Cache coherency protocol REF_CMD bus 144C is one of multiple REF_CMDbusses that PBC 140 uses to communicate with all snoop devices such asprocessor C 130. More specifically, the REF_CMD bus 144C communicates areflection or copy of communication data requests from other masterdevices within IHS 100. In other words, PBC 140 receives data requestcommands from one or multiple master devices and reflects those commandsto one or multiple snoop devices within IHS 100. In response to thereflected command on the REF_CMD bus 144C, processor C 130 returns apartial response on the PART_RESP bus 146C. The partial responsecommunication includes information pertaining to a memory data requestfrom a particular master device. PBC 140 may combine the results ofpartial responses from all snoop devices within IHS 100 and generate acombined response communication. Processor bus controller sends thecombined response communication on the COMB_RESP bus 148C to processor C130.

IHS 100 includes a memory controller 150 that couples to PBC 140 via acommunications interface 145D that includes three cache coherencyprotocol busses, namely a REF_CMD bus 144D, a PART_RESP bus 146D and aCOMB_RESP bus 148D. These cache coherency interface busses, REF_CMD bus144D, PART_RESP bus 146D, and COMB_RESP bus 148D correspond to areflected command bus, a partial response bus and a combined responsebus, respectively. Memory controller 150 couples to a system memory 155that provides random access storage for IHS 100.

An I/O bridge 160 couples to PBC 140 via a communications interface 145Ethat includes four cache coherency protocol busses, namely an INIT_CMDbus 144E, a REF_CMD bus 144E, a PART_RESP bus 146E and a COMB_RESP bus148E. These cache coherency interface busses INIT_CMD bus 142E, REF_CMDbus 144E, PART_RESP bus 146E, and COMB_RESP bus 148E correspond to aninitial command bus, a reflected command bus, a partial response bus anda combined response bus, respectively. I/O bridge 160 couples to an I/Obus controller 170 that allows chip 105 to communicate with other I/Odevices external to chip 105. IHS 100 includes I/O device 180 and I/Odevice 185 that couple to I/O bus controller 170 as shown. I/O device180 and I/O device 185 represent any device external to chip 105 thatmay transfer data, such as a hard drives, USB drives, and DVD drives,for example.

IHS 100 includes four cache coherency bus groups namely, an initialcommand group 142, a reflected command group 144, a partial responsegroup 146, and a combined response group 148. Each bus group includesmultiple conductors with respective signals that communicate primarilyin the direction of the respective arrows, as shown in FIG. 1. Theinitial command bus group 142 includes the INIT_CMD bus 142A, theINIT_CMD bus 142B, the INIT_CMD bus 142C, and the INIT_CMD bus 142E.Memory controller 150 does not employ an initial command bus. Thereflected command bus group 144 includes the REF_CMD bus 144A, theREF_CMD bus 144B, the REF_CMD bus 144C, the REF_CMD bus 144D, and theREF_CMD bus 144E. The partial response bus group 146 includes thePART_RESP bus 146A, the PART_RESP bus 146B, the PART_RESP bus 146C, thePART_RESP bus 146D, and the PART_RESP bus 146E. The combined responsebus group 148 includes the COMB_RESP bus 148A, the COMB_RESP bus 148B,the COMB_RESP bus 148C, the COMB_RESP bus 148D, and the COMB_RESP bus148E. One cache coherency bus group, such as the initial command group142, may include as many as 100 signals or more. Reducing the number ofsignals and interconnects in an IHS is very desirable.

In the example of FIG. 1, IHS 100 employs master devices such asprocessor A 110, processor B 120, processor C 130, and I/O bridge 160.Master devices may initiate memory data requests via the initial commandbus group 142 to communicate a memory data request. IHS 100 alsoincludes slave devices such as system memory 155, slave I/O device 180,slave I/O device 185, or other slave devices (not shown). Slave devicesmay store memory data or other information that a master device mayrequest at any time within IHS 100. Stated alternatively, master devicesstore, send and request data for storage or other use, whereas slavedevices store and/or transfer data in response to a master device'srequest or control.

IHS 100 includes snoop devices, namely processor A 110, processor B 120,processor C 130, memory controller 150, and I/O bridge 160. Snoopdevices include any device capable of storing information that anothermaster device of IHS 100 may make request. Snoop devices utilizereflected command bus group 144, partial response bus group 146, andcombined response bus group 148.

FIG. 2 is a flowchart that depicts process flow in the conventionalcache coherency methodology that IHS 100 employs. In more detail, FIG. 2shows conventional master device and slave device data communicationsthat cooperate in the management of cache memory integrity. Process flowbegins at start block 210. A master device, such as processor A 110 ofIHS 100, initiates a memory data request by generating an initialcommand. For example, processor A 110 may generate a memory data requestwith an initial command on the INIT_CMD bus 142A. Master devicesgenerate data requests, as per block 220.

Master device processor A 110 utilizes the INIT_CMD bus 142A. ProcessorB 120 utilizes the INIT_CMD bus 142B. Processor C 130 utilizes theINIT_CMD bus 142C. Other master device I/O bridge 160 utilizes theINIT_CMD bus 142E. PBC 140 utilizes the initial command bus group 142 asa communication interface to all master devices requesting memory datawithin IHS 100.

Memory controller 150 is not a master device in this particularimplementation and does not generate an initial command or request formemory data therein. Each master device of IHS 100 may initiate a datarequest by generating an initial command signal on a correspondingINIT_CMD bus. The master devices send initial commands on the INIT_CMDbusses to the PBC 140 for interpretation and processing. PBC 140receives and collects all initial commands from master devices of IHS100 and determines which data request to select for processing next, asper block 230. IHS 100 supports request pipelining, namely the abilityto have multiple data requests in process or “in flight” at the sametime. This is particularly important in a multi-tasking environment suchas the multi-processor architecture of IHS 100.

In response to receiving an initial command signal from a particularmaster or snoop device, such as processor A 110 for example, PBC 140sends a reflected command on each reflected command bus group 144 toeach snoop device of IHS 100, as per block 240. Each device of systemIHS that resides on a reflected command bus in reflected command busgroup 144 is a snoop device. Snoop devices may be master devices orother devices within IHS 100 that monitor the address range of aparticular data request by any other master device. If that particulardata request includes a reference to an address range that matches anaddress range within the local cache of the snoop device receiving thereflected command, cache coherency protocols require the snoop device torespond with information about the snoop device's particular memorydata. Stated in another manner, PBC 140 sends a copy of the request froma master device for data to all devices within IHS 100 that may containthat data or manage that data in other devices.

In more detail, PBC 140 sends the reflected command to processor A 110on the REF_CMD bus 144A. PBC 140 sends the reflected command toprocessor B 120 on the REF_CMD bus 144B, and to processor C120 on theREF_CMD bus 144C. PBC 140 sends the reflected command to memorycontroller 150 on the REF_CMD bus 144D and to I/O bridge 160 on theREF_CMD bus 144E.

Each device of IHS 100 that receives the reflected command interpretsthe command or request for memory data and responds with a partialresponse. Each snoop device that receives a reflected command sends apartial response, as per block 250. A snoop device such as processor A110 responds to a reflected command from PBC 140 with a partial responsecommunication on the PART_RESP bus 146A. Processor B 120 responds to areflected command from PBC 140 with a partial response communication onthe PART_RESP bus 146B. Processor C 130 responds to a reflected commandfrom PBC 140 with a partial response communication on the PART_RESP bus146C. Memory controller 150 responds to a reflected command from PBC 140with a partial response communication on the PART_RESP bus 146D. I/Obridge 160 responds to a reflected command from PBC 140 with a partialresponse communication on the PART_RESP bus 146E.

Each partial response from a snoop device to PBC 140 on partial responsebus group 146 takes the form of one of multiple response types. One typeof a partial response from a specific snoop device within IHS 100 is a“retry response”. A retry response instructs PBC 140 to resend thereflected command signal again to that specific snoop device. Such aretry response from the specific snoop device may signal the process buscontroller 140 that the snoop device is busy and cannot respond at thepresent time. A snoop device retry response could be the result of manyconditions, such as waiting for data to settle, the bus is busy, orother reasons.

Another partial response type that a snoop device may send is the“acknowledge” response. Upon receiving an acknowledge response, PBC 140interprets that response as a lack of data availability from the snoopdevice that sends that response. The snoop device may not have the datain its local cache or any other reason for returning an acknowledgeresponse. Different types of snoop device partial responses are known tothose skilled in the art.

To maintain cache coherency, in one implementation PBC 140 interpretseach partial response from each snoop device and combines the responsesinto a special information communication. That special informationcommunication is a combined response communication that utilizes thecombined response bus group 148. PBC 140 utilizes the combined responsebus group 148 to send a combined response communication to each snoopdevice in IHS 100, as per block 260.

Snoop devices or other devices in IHS 100 that maintain memory data forsharing with other devices must maintain an accurate representation ofthat memory data. Maintaining data integrity for cache memory data in adevice such as processor A 110 is known as cache coherency. Cache memoryis a copy of some other memory location in IHS 100. Processor A 110monitors communications on the local cache coherency busses namely, theINIT_CMD bus 142A, the REF_CMD bus 144A, the PART_RESP bus 146A, and theCOMB_RESP bus 148A. Monitoring the data communications on the localcache coherency busses allows processor A 110 to maintain an accuraterepresentation of the memory data within L1 cache 115, and L2 cache 117.For example, L1 cache 115 may contain a copy of particular memory datathat resides in a primary storage location in system memory 155. In thatscenario, L1 cache 115 must be under strict cache coherency control. Thecache coherency protocol method ends, per end block 270.

Processor A 110 may monitor a communication on communications interface145A that indicates a copy of the particular memory data in systemmemory 155 is changing. If that condition occurs, then the data in L1cache 115 is no longer valid. Processor A 110 must modify that memorydata location in L1 cache as invalid. Subsequent requests for that datamust indicate the invalid status of that memory to any other device inIHS 100 that requests that data. At a future time, Processor A 110 mayupdate the memory data it receives from system memory 155 and remove theinvalid reference. In the same manner, other snoop devices of IHS 100maintain their local cache memory store integrity.

One major problem with this approach is that all initiating, or datarequesting, snoop devices must receive and process the combined responsesignals regardless of whether they generate data requests or not. Areduction in cache coherency bus configuration is possible by analyzingthe master or slave status of I/O devices. For example, I/O devices donot typically require full cache coherency protocols. Attachment to thefull array of cache coherency busses is not entirely necessary.Moreover, it is often not desirable for an I/O device to include thecomplexity of the logic required to participate fully in the cachecoherency protocol methodology.

FIG. 3 depicts an information handling system (IHS) 300 with enhancedcache coherency protocol capability. In one embodiment, IHS 300 includesa processor integrated circuit (IC) chip 305 including multipleprocessors and respective multiple caches or cache memories. IHS 300includes a processor A 310 with a processor core 312. Processor core 312is a master processor core since processor A 310 is a master device ofIHS 300. Processor core 312 couples to an L1 cache 315 that couples toan L2 cache 317 within processor A 310. L1 cache 315 may be smaller,namely include less memory, than the L2 cache 317. L1 cache 315 providesprocessor core 312 with local fast access to memory data.

If processor core 312 requests particular memory data from L1 cache 315,and L1 cache 315 returns a “cache hit”, then the particular memory datais available from L1 cache 315. However, if processor core 312 requestsparticular memory data from L1 cache 315, and L1 cache 315 returns a“cache miss”, the particular memory data is not available from L1 cache315. Processor core 312 then continues searching for the particularmemory data by passing the data request through L1 cache 315 into L2cache 317 in an attempt to find the particular memory data. If L2 cache317 returns a “cache miss”, the particular data is not available from L2cache 317. If the particular data is not available from any internalcache of processor A 310, then processor core 312 will initiate a datarequest to memory external to processor A 310 in an attempt to locateand access the particular memory data. That particular memory data mayreside in another processor such as processor B 320 or processor C330,or in system memory 355, or in master I/O device 365 or slave I/O device375, or any other memory location inside or external to IHS 300.Processor A 310 is a master device of IHS 300 with the capability ofinitiating memory data requests.

IHS 300 includes a processor B 320 with a processor core 322. Processorcore 322 couples to an L1 cache 325 that couples to an L2 cache 327within processor B 320. L1 cache 325 may be smaller than L2 cache 327and provides processor core 322 with local fast access to memory data.Processor B 320 is a master device of IHS 300 with the capability ofinitiating memory data requests. IHS 300 also includes a processor C 330that includes a processor core 332. Processor core 332 couples to an L1cache 335 that couples to an L2 cache 337 within processor C 330. L1cache 335 is typically smaller than L2 cache 337 and provides processorcore 332 with fast local access to memory data. Processor C 330 is amaster device of IHS 300 with the capability of initiating memory datarequests.

A processor bus controller (PBC) 340 couples to processor A 310 via acommunications interface 345A that includes four cache coherencyprotocol busses, namely an INIT_CMD bus 342A, a REF_CMD bus 344A, aPART_RESP bus 346A and a COMB_RESP bus 348A. PBC 340 acts as both anarbiter and a gateway for handling data requests in the manner describedin more detail below. Cache coherency protocol INIT_CMD bus 342A is an“initial command” communications bus that a master device such asprocessor A 310 uses to communicate with PBC 340. In particular,processor A 310 uses the INIT_CMD bus 342A to communicate a memory datarequest to PBC 340.

Cache coherency protocol REF_CMD bus 344A is a “reflected command” busthat a bus controller such as PBC 340 utilizes to communicate with asnoop device, namely processor A 310. Snoop devices are any devices thatcommunicate with PBC 340 and that also include a copy of data that amaster device may require. More specifically, the REF_CMD bus 344Acommunicates a reflection or copy of communication data requests fromother master devices within IHS 300. PBC 340 receives data requestcommands from one or multiple master devices and reflects those commandsto one or multiple snoop devices within IHS 300.

In response to the reflected command on the REF_CMD bus 344A, processorA 310 returns a “partial response” on the PART_RESP bus 346A. Thepartial response communication includes information pertaining to amemory data request from a particular master device. For example, thepartial response may be a “retry”, “acknowledge”, or other partialresponse. Different types of snoop device partial responses are known tothose skilled in the art. PBC 340 may combine the results of partialresponses from all snoop devices within IHS 300 and generate a “combinedresponse”. PBC 340 sends the combined response communication on theCOMB_RESP bus 348A to processor A 310.

PBC 340 couples to processor B 320 via a communications interface 345Bthat includes four cache coherency protocol busses, namely an INIT_CMDbus 342B, a REF_CMD bus 344B, a PART_RESP bus 346B and a COMB_RESP bus348B. The cache coherency protocol INIT_CMD bus 342B is an initialcommand communications bus that a master device such as processor B 320uses to communicate with PBC 340. In particular, processor B 320utilizes the INIT_CMD bus 342B to initiate a memory data request todevices external to processor B 320.

Cache coherency protocol REF_CMD bus 344B is a reflected command busthat PBC 340 utilizes to communicate to a snoop device, namely processorB 320. More specifically, the REF_CMD bus 344B contains a reflection orcopy of communication data requests from other master devices within IHS300. In other words, PBC 340 receives data request commands from one ormultiple master devices and reflects those commands to one or multiplesnoop devices within IHS 300.

In response to the reflected command on the REF_CMD bus 344B, processorB 320 returns a partial response on the PART_RESP bus 346B. The partialresponse communication includes information pertaining to a memory datarequest from a particular master device. PBC 340 may combine the resultsof partial responses from all snoop devices within IHS 300 and generatea combined response. Processor bus controller PBC 340 sends the combinedresponse communication on the COMB_RESP bus 348B to processor B 320.

PBC 340 couples to processor C 330 via a communications interface 345Cthat includes four cache coherency protocol busses namely, an INIT_CMDbus 342C, a REF_CMD bus 344C, a PART_RESP bus 346C and a COMB_RESP bus348C. The cache coherency protocol INIT_CMD bus 342C is an initialcommand communications bus that a master device such as processor C 330uses to communicate with PBC 340. In particular, processor C 330 usesthe INIT_CMD bus 342C to communicate a memory data request to devicesexternal to processor C 330.

Cache coherency protocol REF_CMD bus 344C is a reflected command busthat PBC 340 utilizes to communicate with a snoop device, namelyprocessor C 330. More specifically, the REF_CMD bus 344C communicates areflection or copy of communication data requests from other masterdevices within IHS 300. PBC 340 receives data request commands from oneor multiple master devices and reflects those commands to one ormultiple snoop devices within IHS 300.

In response to the reflected command on the REF_CMD bus 344C, processorC 330 returns a partial response on the PART_RESP bus 346C. The partialresponse communication includes information pertaining to a memory datarequest from a particular master device. PBC 340 may combine the resultsof partial responses from all snoop devices within IHS 300 and generatea combined response. Processor bus controller sends the combinedresponse communication on the COMB_RESP bus 348C to processor C 330.

IHS 300 includes a memory controller 350 that couples to PBC 340 via acache coherency protocol communications interface 345D. Interface 345Dincludes a REF_CMD bus 344D, a PART_RESP bus 346D, and a COMB_RESP bus348D. These cache coherency interface busses REF_CMD bus 344D, PART_RESPbus 346D, and COMB_RESP bus 348D respectively communicate the reflectedcommand, partial response and combined response. These interface busses344D, 346D, and 348D form the cache coherency protocol communicationsinterface 345D of PBC 340 to memory controller 350. Memory controller350 couples to a system memory 355.

PBC 340 couples to a master I/O device controller 360 via acommunications interface 345E that includes two cache coherency protocolbusses namely, an INIT_CMD bus 342E and a COMB_RESP bus 348E. Master I/Odevice controller 360 couples to a master I/O device 365. The cachecoherency protocol INIT_CMD bus 342E is an initial commandcommunications bus that a master device controller such as master I/Odevice controller 360 uses to communicate with PBC 340. Master I/Odevice controller 360 uses the INIT_CMD bus 342E to communicate a memorydata request to any memory within IHS 300. Master I/O device 365 mayinclude a local on-board memory store 362 that may not take the form ofa cache. PBC 340 couples to a slave I/O device controller 370 via acommunications interface 345F that includes two cache coherency protocolbusses, namely a REF_CMD bus 344F and a PART_RESP bus 346F. Slave I/Odevice controller 370 couples to a slave I/O device 375.

The cache coherency protocol REF_CMD bus 344F is a reflected commandcommunications bus that PBC 340 uses to send copies of master devicememory data requests to slave I/O device 375. The cache coherencyprotocol PART_RESP bus 346F is a partial response communication bus thatslave I/O device controller 370 utilizes to communicate with PBC 340.More particularly, in response to reflected commands, slave I/O devicecontroller 370 communicates with PBC 340 via the PART_RESP bus 346F.Master I/O device 365 is an external I/O device capable of storing andretrieving memory data information. An external device is a deviceoutside of processor IC chip 305. A network server and other similarstorage devices are examples of master I/O devices. Slave I/O device 375represents any device external to processor IC chip 305 that may storeand transfer data, for example a hard drive, USB drive, a DVD drive,etc. In other embodiments, more I/O devices (not shown) may connect toPBC 340 in a similar manner to the way master I/O device 365 and slaveI/O device 375 connect to PBC 340. As shown in FIG. 3, both master I/Odevice 365 and slave I/O device 375 couple indirectly to PBC 340 viamaster I/O device controller 360 and slave I/O device controller 370,respectively. Master I/O device controller 360 and slave I/O devicecontroller 370 couple to PBC 340 via interface busses 345E and 345F,respectively. System memory 355, which is not a master device, couplesto PBC 340 indirectly via memory controller 350.

IHS 300 includes four cache coherency bus groups, namely the initialcommand group 342, the reflected command group 344, the partial responsegroup 346, and the combined response bus group 348. Each bus grouprepresents multiple conductors with multiple signals primarilytransmitting in the directions of the arrows as shown in FIG. 3 at theedge of PBC 340. The initial command bus group 342 includes the INIT_CMDbus 342A, the INIT_CMD bus 342B, the INIT_CMD bus 342C, and the INIT_CMDbus 342E. Memory controller 350 and slave I/O device controller 370 donot employ an initial command bus.

The reflected command bus group 344 includes the REF_CMD bus 344A, theREF_CMD bus 344B, the REF_CMD bus 344C, the REF_CMD bus 344D, and theREF_CMD bus 344F. Master I/O device 365 does not employ a reflectedcommand bus. The partial response bus group 346 includes the PART_RESPbus 346A, the PART_RESP bus 346B, the PART_RESP bus 346C, the PART_RESPbus 346D, and the PART_RESP bus 346F. Master I/O device 365 does notemploy a partial response bus. The combined response bus group 348includes the COMB_RESP bus 348A, the COMB_RESP bus 348B, the COMB_RESPbus 348C, the COMB_RESP bus 348D, and the COMB_RESP bus 348E. Slave I/Odevice controller 370 does not employ a combined response bus.

In the embodiment of FIG. 3, IHS 300 employs master devices, namelyprocessor A 310, processor B 320, processor C 330, and master I/O device365. Master devices may initiate memory data requests utilizing theinitial command bus group 342 to communicate such a memory data requestto other master devices or snoop devices of IHS 300. IHS 300 includesslave devices, namely system memory 355, slave I/O device 375, and anyother slave devices (not shown). Slave devices may store memory data orother information that a master device may request at any time withinIHS 300. In other words, master devices store, send and request data forstorage or other use, and slave devices store and/or transfer data inresponse to a master devices request or control.

IHS 300 includes snoop devices, namely processor A 310, processor B 320,processor C 330, and memory controller 350. A snoop device is any devicecapable of storing information that another master device of IHS 300 mayrequest. Snoop devices utilize the initial command bus group 342, thereflected command bus group 344, the partial response bus group 346, andthe combined response bus group 348 to communicate with PBC 340.

IHS 300 supports request pipelining, namely the ability to have multipledata request in process or “in flight” at the same time. This isparticularly important in a multi-tasking environment such as themulti-processor architecture of IHS 300. One method of managing multipledata requests “in flight” is a “serialization technique”. Theserialization technique blocks the progress of data requests by forcingdata request retries. For example, PBC 340 generates data requestretries on any data request trying to access or modify memory data froma current “in flight” data request. IHS 300 may use an adjacent addressmap protection or physical address attribute map (PAAM) scheme toaccomplish serialization. One such serialization methodology isdisclosed in U.S. Pat. No. 6,405,289 entitled “Multiprocessor System InWhich A Cache Serving As A Highest Point Of Coherency Is Indicated By ASnoop Response”, inventors Arimilli, et al., the disclosure of which isincorporated herein by reference in its entirety. PAAM or “coherencytriangle” protection techniques effectively block any data requests thathave an address that matches the address of a data request in flight.

FIG. 4 is a flowchart that depicts process flow in IHS 300 with enhancedcache coherency protocol capability. In more detail, FIG. 4 shows amethodology wherein master devices, slave devices, and snoop devicescommunicate data in a manner that manages and preserves cache memoryintegrity. Process flow begins at start block 405. A master device, suchas processor A 310 of IHS 300, initiates a data request by generating aninitial command. For example, processor A 310 may generate a particularmemory data request with an initial command on the INIT_CMD bus 342A.The particular data request from processor A 310 includes a referenceaddress or address range of the memory data request. For example, theparticular memory data request may reference an address range of systemmemory 355, an address range of slave I/O device 375, or any other datamemory location.

Master devices generate data requests by transmitting respective initialcommands, as per block 410. Master device processor A 310 utilizes theINIT_CMD bus 342A, processor B 320 utilizes the INIT_CMD bus 342B, andprocessor C 330 utilizes the INIT_CMD bus 342C to transmit such datarequests to PBC 340. Another master I/O device controller 360 utilizesthe INIT_CMD bus 342E to transmit a data request. PBC 340 uses theinitial command bus group 342 as a communication interface with allmaster devices requesting memory data within IHS 300. Master devicesgenerating data requests include the address or referenced address rangeof the data request within the initial command data request. Thereferenced address range may reference an address range in system memory355.

In the disclosed embodiment, memory controller 350 is not a masterdevice and does not generate an initial command or data request therein.Each master device of IHS 300 may initiate a data request by generatinga respective initial command signal on a respective initial command bus.Master devices send initial commands on the initial command bus group342 to PBC 340 for interpretation and processing.

PBC 340 receives and collects all data requests as initial commands fromthe master devices of IHS 300. PBC 340 determines if the next datarequest to process is a data request to a slave I/O device or a datarequest from a master I/O device, as per decision block 420. PBC 340interprets the data request or initial command communication addressrange to determine the memory location of the data within IHS 300. Ifthe data request is to a slave I/O device or from a master I/O device,then PBC 340 modifies the data request as “non-cacheable”, as per block425. Another term for non-cacheable, is “no intent to cache” by therequesting master device. Non-cacheable refers to any data request froma master device for which PBC 340 determines the data is not valid forcaching in any cache within IHS 300.

After the data request modification of step 425, or if the data requestis not to a slave I/O or from a master I/O device, PBC 340 collects allmaster data requests and selects the next data request for processing,as per block 430. A significant aspect of cache coherency protocols asshown in this embodiment is the ability of IHS 300 to manage multipledata requests for information from the same address location. Forexample while one master device, such as processor A 310 sends data tosystem memory 355 at a particular address, processor B 320 may requestthe same data from that same particular address location. The enhancedcache coherency protocol method as described in FIG. 4 demonstrates onemethodology to avoid such potential cache conflicts in the example ofIHS 300. A data request from any master device in IHS 300 that ispending completion is an “in flight” data request. PBC 340 blocks thedata request per step 430 if another data request “in flight” has thesame address, as per block 435.

PBC 340 sends a reflected command on the reflected command 344 busgroup, as per block 440. More specifically, PBC 340 sends the reflectedcommand to each snoop device of IHS 300. Each device of IHS 300 thatresides on a reflected command bus, namely reflected command bus group344, is a snoop device or snooper. Snoop devices may be master devicesor other devices within IHS 300 that monitor the address range of aparticular data request by any other master device. If that particulardata request includes a reference to an address range that matches anaddress range within the local cache of the snoop device receiving thereflected command, cache coherency protocols require the snoop device torespond with information about the snoop device's particular memorydata. Stated alternatively, PBC 340 sends a copy of the request from amaster device for particular data to all devices within IHS 300 that maycontain or manage that particular data. In more detail, PBC 340 sendsthe reflected command to processor A 310 on the REF_CMD bus 344A. PBC340 sends the reflected command to processor B 320 on the REF_CMD bus344B, and to processor C 330 on the REF_CMD bus 344C. PBC 340 sends thereflected command to memory controller 350 on the REF_CMD bus 344D andto slave I/O device controller 370 on the REF_CMD bus 344F.

Each device of IHS 300 that receives the reflected command on arespective reflected command 344 bus group is a snoop device, Each snoopdevice interprets the data request that processor bus controller 340reflects and responds with a partial response, as per block 450. A snoopdevice within IHS 300 responds to a reflected command from PBC 340 witha partial response. For example, a snoop device such as processor A 310responds to a reflected command from PBC 340 with a partial responsecommunication on the PART_RESP bus 346A. A snoop device such asprocessor B 320 responds to a reflected command from PBC 340 with apartial response communication on the PART_RESP bus 346B. A snoop devicesuch as processor C 330 responds to a reflected command from PBC 340with a partial response communication on the PART_RESP bus 346C. A snoopdevice such as memory controller 350 responds to a reflected commandfrom PBC 340 with a partial response communication on the PART_RESP bus346D. A snoop device such as slave I/O device controller 370 responds toa reflected command from PBC 340 with a partial response communicationon the PART_RESP bus 346E.

In one embodiment, master devices handle all data requests to slave I/Odevices as non-cacheable. In one embodiment, PBC 340 ensures that thecombined response for all data requests to slave I/O devices isdependent only on the partial response of the slave I/O device itself.For example, this allows slave I/O device controller 370 to assume thatthe combined response is the same as the partial response that slave I/Odevice controller 370 provides to PBC 340. With these conditions met,slave I/O device controller 370 or any other slave I/O devices do notrequire a combined response bus. PBC 340 can determine which devices areslave I/O devices by hardware strapping techniques, software deviceaddressing data, or other hardware or software techniques. The partialresponse from any slave I/O device may include a decode signal that aparticular slave I/O device asserts when the address of a data requestmatches the particular slave I/O memory data address range. PBC 340 maycompare the decode signal from a known list of slave I/O devices. If thedecode signal matches a slave I/O device of IHS 300, then PBC 340 blocksthe partial responses from all other devices.

Each partial response from a snoop device to the PBC 340 on the partialresponse bus group 346 may take the form of multiple response types. Onetype of a partial response from a specific snoop device within IHS 300is a “retry response”. A retry response instructs PBC 340 to resend thereflected command signal again to that specific snoop device. Such aretry response from the specific snoop device may instruct the processbus controller 340 that the specific snoop device is busy and cannotrespond currently.

A snoop device retry response may be the result of multiple conditions,such as waiting for data to settle, data bus busy, or other reasons.Another retry response type from a snoop device may be an “acknowledge”response. Upon receiving an acknowledge response, PBC 340 interpretsthat response as an allowable retry from the snoop device sending thatresponse. The sending snoop device has the data in cache in a state thatdoes not conflict with the data request, thus returning an acknowledgeresponse to allow the data request transaction to proceed.

Snoop devices may include a decode signal with the partial responsecommunication. PBC 340 receives partial response communications alongwith any decode signal information. PBC 340 tests to determine if thedecode signal is from a slave I/O device such as slave I/O device 375,as per decision block 460. If the test determines that the decode signalis from a slave I/O device, then PBC 340 gates off or blocks partialresponses from all other devices of IHS 300, as per block 465. Howeverif the test determines that the decode signal is not from a slave I/Odevice, then PBC 340 combines all partial responses, as per block 470.

To maintain cache coherency, PBC 340 interprets each partial responsefrom each snoop device and combines the responses into a special type ofcommunication, namely a combined response. The combined responsecommunication utilizes the combined response bus group 348. PBC 340sends the combined response on the combined response 348 bus group, asper block 480. In response to receiving the combined response, masterdevices such as a master processor A 310 or master I/O device 365perform local updates of their respective cache memories, as per block485. The enhanced cache coherency protocol method ends at end block 490.

Snoop devices or other devices in IHS 300 that maintain memory data forsharing with other devices must also maintain an accurate representationof that memory data. Maintaining data integrity for cache memory data ina device such as processor A 310 is important for cache coherency. Cachememory such as L1 cache 315 and L2 cache 317 contain a copy of someother memory location in IHS 300. Any master device or device with cachememory in IHS 300 should maintain an accurate representation of datawithin their respective local caches. For example, processor A 310monitors communications on the local cache coherency busses 345A, namelythe INIT_CMD bus 342A, the REF_CMD bus 344A, the PART_RESP bus 346A, andthe COMB_RESP bus 348A. Monitoring the data communications on the localcache coherency busses allows processor A 310 to maintain an accuraterepresentation of the memory data within L1 cache 315 and L2 cache 317.If the L1 cache 315 contains a copy of particular memory data for whichsystem memory 355 is the primary storage location, that particularmemory data should be under cache coherency control.

Processor A 310 may monitor a communication of cache coherency busses345A that indicates a change to particular data in system memory 355,thus rendering invalid a local copy of the particular data in L1 cache315. Processor A 310 should thus modify that particular data location inL1 cache 315 as invalid. Subsequent requests for that particular datashould indicate the invalid status of that particular data to any otherdevice in IHS 300 that requests that particular data. During futureoperations of IHS 300, processor A 310 may update the particular data inL1 cache 315 and remove the invalid reference. In the same manner, othersnoop devices of IHS 300 maintain their local cache memory storeintegrity.

Modifications and alternative embodiments of this invention will beapparent to those skilled in the art in view of this description of theinvention. Accordingly, this description teaches those skilled in theart the manner of carrying out the invention and is intended to beconstrued as illustrative only. The forms of the invention shown anddescribed constitute the present embodiments. Persons skilled in the artmay make various changes in the shape, size and arrangement of parts.For example, persons skilled in the art may substitute equivalentelements for the elements illustrated and described here. Moreover,persons skilled in the art after having the benefit of this descriptionof the invention may use certain features of the invention independentlyof the use of other features, without departing from the scope of theinvention.

1. A method of maintaining cache coherency in a processor integratedcircuit (IC), the method comprising: providing the processor IC with aplurality of master processor cores that each communicate with arespective cache memory; providing a processor bus controller (PBC) thatcouples to the master processor cores, the PBC being configured tocouple to a system memory indirectly via a memory controller, the PBCbeing further configured to couple indirectly to a master I/O device anda slave I/O device via a respective master I/O device controller andslave I/O device controller; sending to the PBC, by a master processorcore or a master I/O device, an initial command data request thatreferences an address range in the system memory, thus determining areferenced address range; receiving, by the PBC, the initial commanddata request and in response sending a reflected command to any masterprocessor core, system memory and I/O slave device that the PBCdetermines to be within the referenced address range; sending inresponse to the reflected command, by the master processor cores, systemmemory, and slave I/O device in the referenced address range, respectivepartial responses to the PBC; and blocking, by the PBC, the partialresponse of the memory controller, the master processor cores, and themaster I/O device but not the slave I/O device from inclusion in acombined response that the PBC sends to master processor cores, masterI/O devices and system memory.
 2. The method of claim 1, wherein thepartial response of the slave I/O device includes a decode signal thatindicates the slave I/O device is a slave I/O device.
 3. The method ofclaim 1, further comprising receiving, by the plurality of masterprocessor cores and the master I/O device controller, the combinedresponse.
 4. The method of claim 3, further comprising updating, by theplurality of master processor cores, the respective cache memories ofthe processor cores in response to the combined response.
 5. The methodof claim 3, further comprising updating, by the master I/O device amemory of the master I/O device in response to the combined response. 6.The method of claim 1, further comprising determining, by the PBC, if aninitial command data request is directed to a slave I/O device or from amaster device that is a master I/O device, thus identifying an I/Orequest.
 7. The method of claim 6, further comprising designating, bythe PBC, the initial command data request as a non-cacheable request ifthe determining step determines that the initial command data request isidentified as an I/O request.
 8. The method of claim 1, wherein thepartial response of at least one of the plurality of master processorcores, the memory controller and the slave I/O device controller is aretry response.
 9. A processor integrated circuit, comprising: aplurality of master processor cores; a plurality of cache memories, eachcache memory being coupled to a respective master processor core; and aprocessor bus controller (PBC) coupled to the plurality of masterprocessor cores, the PBC being configured to couple to a system memoryindirectly via a memory controller, the PBC being further configured tocouple directly to a master I/O device controller and a slave I/O devicecontroller, wherein the PBC receives an initial command data requestfrom a master processor core or a master I/O device, the initial commanddata request including a referenced address range in the system memory,and in response to the initial command data request sending, by the PBC,a reflected command to any master processor core, system memory and I/Oslave device that the PBC determines to be within the referenced addressrange; wherein in response to the reflected command, the masterprocessor cores, system memory and slave I/O device in the referencedaddress range, send respective partial responses to the PBC, such thatthe PBC blocks the partial response of the memory controller, the masterprocessor cores, and the master I/O device but not the slave I/O devicefrom inclusion in a combined response that the PBC sends to masterprocessor cores, master I/O devices and system memory.
 10. The processorintegrated circuit of claim 9, wherein the partial response of the slaveI/O device includes a decode signal that indicates the slave I/O deviceis a slave I/O device.
 11. The processor integrated circuit of claim 9,wherein the plurality of master processor cores and the master I/Odevice controller are configured to receive the combined response. 12.The processor integrated circuit of claim 11, wherein the masterprocessor cores update their respective cache memories in response tothe combined response.
 13. The processor integrated circuit of claim 11,wherein the master I/O device updates a memory of the master I/O devicein response to the combined response.
 14. The processor integratedcircuit of claim 9, wherein the PBC determines if an initial commanddata request is directed to a slave I/O device or from a master devicethat is a master I/O device, thus identifying an I/O request.
 15. Theprocessor integrated circuit of claim 14, wherein the PBC designates theinitial command data request as a non-cacheable request if thedetermining step determines that the initial command data request isidentified as an I/O request.
 16. The processor integrated circuit ofclaim 9, wherein the partial response of at least one of the pluralityof master processor cores, the memory controller and the slave I/Odevice controller is a retry response.
 17. An information handlingsystem (IHS), comprising: a system memory; a processor integratedcircuit, coupled to the system memory, the processor integrated circuitincluding: a plurality of master processor cores; a plurality of cachememories, each cache memory being coupled to a respective masterprocessor core; and a processor bus controller (PBC) coupled to theplurality of master processor cores, the PBC being configured to coupleto the system memory indirectly via a memory controller, the PBC beingfurther configured to couple directly to a master I/O device controllerand a slave I/O device controller, wherein the PBC receives an initialcommand data request from a master processor core or a master I/Odevice, the initial command data request including a referenced addressrange in the system memory, and in response to the initial command datarequest sending, by the PBC, a reflected command to any master processorcore, system memory and I/O slave device that the PBC determines to bewithin the referenced address range; wherein in response to thereflected command, the master processor cores, system memory and slaveI/O device in the referenced address range, send respective partialresponses to the PBC, such that the PBC blocks the partial response ofthe memory controller, the master processor cores, and the master I/Odevice but not the slave I/O device from inclusion in a combinedresponse that the PBC sends to master processor cores, master I/Odevices and system memory.
 18. The IHS of claim 17, wherein the partialresponse of the slave I/O device controller includes a decode signalthat indicates the slave I/O device is a slave I/O device.
 19. The IHSof claim 17, wherein the plurality of master processor cores and themaster I/O device controller are configured to receive the combinedresponse.
 20. The IHS of claim 19, wherein the plurality of masterprocessor cores update their respective cache memories in response tothe combined response.
 21. The IHS of claim 19, wherein the master I/Odevice updates a memory of the master I/O device in response to thecombined response.
 22. The IHS of claim 17, wherein the PBC determinesif an initial command data request is directed to a slave I/O device orfrom a master device that is a master I/O device, thus identifying anI/O request.
 23. The IHS of claim 22, wherein the PBC designates theinitial command data request as a non-cacheable request if thedetermining step determines that the initial command data request isidentified as an I/O request.
 24. The IHS of claim 17, wherein thepartial response of at least one of the plurality of master processorcores, the memory controller and the slave I/O device controller is aretry response.